Objective lens

ABSTRACT

There is provided an objective lens used for a plurality of types of optical discs having a front surface and a rear surface, each of which includes an inner region and an outer region. The outer region has a surface shape which suppresses a coma caused when a beam used for a first optical disc is incident thereon obliquely with respect to an optical axis of the objective lens. The inner region is configured such that, at a boundary position between the inner region and the outer region, the coma caused when a beam used for a second optical disc is incident on the inner region obliquely at a first angle with respect to the optical axis is less than the coma caused when the beam used for the second optical disc is incident on the outer region obliquely at the first angle with respect to the optical axis. Further, an inclination θ 2A  of the inner region and an inclination θ 2B  of the outer region of the rear surface satisfy a condition: −2.5&lt;θ 2B −θ 2A &lt;0.0 . . . (1).

This application is a divisional of pending U.S. patent application Ser.No. 10/900,250, filed Jul. 28, 2004, the disclosure of which isexpressly incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

The present invention relates to an objective lens used for an opticaldisc drive, which is capable of recording data to and/or reproducingdata from a plurality of types of optical discs having differentrecording densities and having different thicknesses of cover layers.

There are various types of optical discs on which digital information isrecorded at various densities, respectively. For example, a DVD (digitalversatile disc) has a recording density higher that that of a CD(compact disc) or a CD-R (CD Recordable) and has a cover layer thinnerthan that of the CD or CD-R.

When recording/reproducing operation for the DVD having a higherrecording density is performed, a smaller beam spot diameter is requiredon a data recording layer of the DVD relative to a beam spot used forthe CD having a lower recording density. For this reason, the opticaldisc drive is configured such that an NA (numerical aperture) is changedto a higher value to obtain a smaller beam spot diameter when the DVD isused and that the NA is changed to a lower value to obtain a larger beamspot diameter when the CD or CD-R is used.

A condition of a spherical aberration in an optical system of theoptical disc drive changes depending on a thickness of the cover layerof the optical disc being used. Therefore, it is required to correct thespherical aberration caused when the optical disc is changed to anotherone having different thickness of the cover layer.

The diameter of the beam spot decreases as a wavelength of a laser beamincident on the objective lens decreases. Therefore, in general, a laserbeam having a wavelength ranging from 635 nm to 660 nm is used for theDVD, and a laser beam having a wavelength ranging from 780 nm to 830 nmis used for the CD. By using such wavelengths, a relatively small beamspot is obtained when the DVD is used, and a relatively large beam spotis obtained when the CD is used.

In general, the optical disc drive is provided with a light sourceconfigured to emit laser beams having different wavelengths in order tosupport different types of optical discs.

Japanese Patent Provisional Publication No. 2001-243651 discloses anobjective lens configured to suitably converge incident laser beams ontodata recording layers of a plurality of types of optical discs havingdifferent thicknesses of cover layers, respectively. On one of lenssurfaces of the objective lens disclosed in this publication, adiffracting structure having a plurality of ring-shaped minute steps isformed. In an optical system disclosed in the publication, each of twocollimated laser beams having different wavelengths is incident on theobjective lens.

In order to form beam spots more suitable for recording data to and/orreproducing data from the plurality of types of the optical discs, theobjective lens is required to be corrected for a coma as well as thespherical aberration. The coma is caused when off-axis light is incidenton the objective lens (i.e., when a beam is incident on the objectivelens obliquely with respect to an optical axis of the objective lens).

However, the objective lens disclosed in the publication 2001-243651 cannot suitably correct the comas for all of the plurality of types ofoptical discs. More specifically, the objective lens disclosed in thepublication is configured to adjust balance of the coma considering theintended use of an optical disc drive including the objective lens.

Japanese Patent Provisional Publication No. 2003-156682 discloses anobjective lens configured to support the plurality of types of theoptical discs and to reduce the coma caused when off-axis light isincident thereon. Each of lens surfaces of this objective lens has aninner region and an outer region located outside the inner region.

The inner region is a region for attaining an NA for a second opticaldisc having a lower recording density. The outer region is a region forattaining an NA for a first optical disc having a recording densityhigher than that of the second optical disc. The inner region and theouter region of each of the lens surfaces of the objective lens havedifferent shapes.

In the publication, it is described that the coma caused when theoff-axis light is incident on the objective lens is reduced by the abovementioned structure. However, in the publication No. 2003-156682, noexplanation is made on how to configure the shapes of the inner regionand the outer region of each lens surface to effectively reduce thecoma.

SUMMARY OF THE INVENTION

The present invention is advantageous in that it provides an objectivelens which is capable of suppressing a coma caused when a beam isincident on the objective lens obliquely with respect to an optical axisof the objective lens so that the objective lens can form beam spotssuitable for recording data to and/or reproducing data from a pluralityof types of optical discs.

According to an aspect of the invention, there is provided an objectivelens used for recording data to and/or reproducing data from a pluralityof types of optical discs having different thicknesses of cover layers.The plurality of types of optical discs includes a first optical discand a second optical disc having a cover layer thicker than that of thefirst optical disc. The objective lens includes a front surface locatedon a light source side, and a rear surface located on an optical discside. Each of the front and rear surface includes an inner region forattaining a numerical aperture required to record data to and/or toreproduce data from the second optical disc, and an outer region forattaining a numerical aperture required to record data to and/or toreproduce data from the first optical disc, the outer region beinglocated outside the inner region.

In this structure, the outer region of each of the front and rearsurfaces has a surface shape which suppresses a coma caused when a beamused for the first optical disc is incident thereon obliquely withrespect to an optical axis of the objective lens. The inner region ofeach of the front and rear surfaces is configured such that, at aboundary position between the inner region and the outer region, thecoma caused when a beam used for the second optical disc is incident onthe inner region obliquely at a first angle with respect to the opticalaxis is less than the coma caused when the beam used for the secondoptical disc is incident on the outer region obliquely at the firstangle with respect to the optical axis.

Further, the rear surface is configured to satisfy a condition:

−2.5<θ_(2B)−θ_(2A)<0.0  (1)

where θ_(2A) (degree) represents an inclination of the inner region ofthe rear surface at the boundary position of the rear surface, θ_(2B)(degree) represents an inclination of the outer region of the rearsurface at the boundary position of the rear surface, the inclinationθ_(2A) is an angle formed by a line normal to the inner region withrespect to the optical axis, the inclination θ_(2B) an angle formed by aline normal to the outer region with respect to the optical axis, andthe inclinations θ_(2A) and θ_(2B) are plus when the inclinations θ_(2A)and θ_(2B) are measured in a clockwise direction with respect to theoptical axis.

With this configuration, the coma can be sufficiently corrected for eachof the plurality of types of optical discs. Consequently, beam spotssuitable for recording data to and/or reproducing data from theplurality of types of optical discs are formed on data recording layersof the plurality of types of optical discs, respectively.

Optionally, the front surface may be configured to satisfy a condition:

−1.2<θ_(1B)−θ_(1A)<0.0  (2)

where θ_(1A) (degree) represents an inclination of the inner region ofthe front surface at the boundary position of the front surface, θ_(1B)(degree) represents an inclination of the outer region of the frontsurface at the boundary position of the front surface, the inclinationθ_(1A) is an angle formed by a line normal to the inner region withrespect to the optical axis, the inclination θ_(1B) is an angle formedby a line normal to the outer region with respect to the optical axis,and the inclinations θ_(1A) and θ_(1B) are plus when the inclinationsθ_(1A) and θ_(1B) are measured in a clockwise direction with respect tothe optical axis.

Still optionally, at least one of the front and rear surfaces may have adiffracting structure.

Still optionally, the objective lens may be used so that magnificationsfor the first and second optical discs are substantially the same.

Still optionally, the outer region of the rear surface may be defined asa region through which a beam passed through the outer region of thefront surface passes.

Still optionally, the inner regions of the front and rear surfaces maybe configured such that the coma is sufficiently reduced for an opticaldisc whose cover layer has an intermediate thickness between the firstoptical disc and the second optical disc.

Still optionally, the rear surface may be configured to satisfy acondition: −2.50<θ_(2B)−θ_(2A)<−0.05 . . . (4). In this case, the rearsurface may be configured to satisfy a condition:

−1.0×10⁻³ <X _(B) −X _(A)<1.0×10⁻³  (3)

where X_(A) represents a distance, measured at the boundary position,between a surface defined by a surface shape of the inner region and aplane tangential to the rear surface at the optical axis, and X_(B)represents a distance, measured at the boundary position, between asurface defined by a surface shape of the outer region and a planetangential to the rear surface at the optical axis.

According to another aspect of the invention, there is provided anobjective lens used for recording data to and/or reproducing data from aplurality of types of optical discs having different thicknesses ofcover layers. The plurality of types of optical discs includes a firstoptical disc and a second optical disc having a cover layer thicker thanthat of the first optical disc. The objective lens includes a frontsurface located on a light source side, and a rear surface located on anoptical disc side. Each of the front and rear surface includes an innerregion for attaining a numerical aperture required to record data toand/or to reproduce data from the second optical disc, and an outerregion for attaining a numerical aperture required to record data toand/or to reproduce data from the first optical disc, the outer regionbeing located outside the inner region.

In this structure, the outer region of each of the front and rearsurfaces has a surface shape which suppresses a coma caused when a beamused for the first optical disc is incident thereon obliquely withrespect to an optical axis of the objective lens. The inner region ofeach of the front and rear surfaces is configured such that, at aboundary position between the inner region and the outer region, thecoma caused when a beam used for the second optical disc is incident onthe inner region obliquely at a first angle with respect to the opticalaxis is less than the coma caused when the beam used for the secondoptical disc is incident on the outer region obliquely at the firstangle with respect to the optical axis. Further, the inner region andthe outer region of the rear surface are continuously connected to eachother at the boundary position of the rear surface.

With this configuration, the coma can be sufficiently corrected for eachof the plurality of types of optical discs. It is preferable that theboundary position between the inner region and the outer region issmoothed, for example, in a grinding process so that ill effects causedby a step formed at the boundary position can be avoided. For thisreason, the boundary position is smoothed.

FIG. 20 schematically shows a cross section of the rear surface of theobjective lens. According to the above mentioned configuration of theobjective lens, the inner region LA₂ and the outer region LB₂ of therear surface are continuously connected to each other as indicated by acurve LA₂−LC₂−LB₂.

Optionally, a size of an area for continuously connecting the innerregion to the outer region may be less than or equal to 2% of a sum ofareas of the inner and outer regions of the rear surface.

The smoothed portion LC₂ may cause reduction in light quantity becauselight passed through the smoother portion LC₂ does not suitably convergeon to the optical disc. To avoid such a phenomenon, according to theabove configuration, the size of the area (smoothed portion) isconfigured to be less than or equal to 2% of the sum of areas of theinner and outer regions of the rear surface.

According to another aspect of the invention, there is provided anobjective lens used for recording data to and/or reproducing data from aplurality of types of optical discs having different thicknesses ofcover layers. The plurality of types of optical discs includes a firstoptical disc and a second optical disc having a cover layer thicker thanthat of the first optical disc. The objective lens includes a frontsurface located on a light source side, and a rear surface located on anoptical disc side. Each of the front and rear surface includes an innerregion for attaining a numerical aperture required to record data toand/or to reproduce data from the second optical disc, and an outerregion for attaining a numerical aperture required to record data toand/or to reproduce data from the first optical disc, the outer regionbeing located outside the inner region.

In this structure, the outer region of each of the front and rearsurfaces has a surface shape which suppresses a coma caused when a beamused for the first optical disc is incident thereon obliquely withrespect to an optical axis of the objective lens. The inner region ofeach of the front and rear surfaces is configured such that, at aboundary position between the inner region and the outer region, thecoma caused when a beam used for the second optical disc is incident onthe inner region obliquely at a first angle with respect to the opticalaxis is less than the coma caused when the beam used for the secondoptical disc is incident on the outer region obliquely at the firstangle with respect to the optical axis.

Further, the objective lens is formed by injection molding using a moldconfigured such that a portion corresponding to the boundary position ofat least one of the front and rear surfaces is processed to be acontinuous surface by using an R-bite.

With this configuration, the coma can be sufficiently corrected for eachof the plurality of types of optical discs. Such process for making theboundary position the continuous surface may be performed if designshapes of the inner region and the outer region of at least one of thefront and rear surfaces are discontinuously connected to each other atthe boundary position or are not completely continuously connected toeach other at the boundary position.

According to another aspect of the invention, there is provided anobjective lens including a front surface and a rear surface. At leastone surface of the front and rear surfaces includes a plurality ofregions having different shapes. The at least one surface is configuredsuch that, at each of boundary positions between adjacent ones of theplurality of regions, the adjacent ones of the plurality of regions arecontinuously connected to each other.

With this configuration, it becomes possible to sufficiently suppressthe coma for each of the plurality of types of optical discs.

In a particular case, the plurality of regions may include an innerregion and an outer region outside the inner region. A size of an areafor continuously connecting the inner region to the outer region is lessthan or equal to 2% of a sum of areas of the inner and outer regions ofthe rear surface.

According to another aspect of the invention, there is provided anoptical system which includes an objective lens having a front surfaceand a rear surface. At least one surface of the front and rear surfacesincludes a plurality of regions having different shapes. The at leastone surface is configured such that, at each of boundary positionsbetween adjacent ones of the plurality of regions, the adjacent ones ofthe plurality of regions are continuously connected to each other.

With this configuration, it becomes possible to sufficiently suppressthe coma for each of the plurality of types of optical discs.

In a particular case, the plurality of regions of the objective lens mayinclude an inner region and an outer region outside the inner region. Asize of an area for continuously connecting the inner region to theouter region is less than or equal to 2% of a sum of areas of the innerand outer regions of the rear surface.

According to another aspect of the invention, there is provided anobjective lens including a front surface and a rear surface. At leastone surface of the front and rear surfaces includes a plurality ofregions having different shapes. The objective lens is formed byinjection molding using a mold configured such that portionscorresponding to boundary positions of adjacent ones of the plurality ofregions are processed to be continuous surfaces by using an R-bite.

With this configuration, it becomes possible to sufficiently suppressthe coma for each of the plurality of types of optical discs. Suchprocess for making the boundary position the continuous surface may beperformed if design shapes of the plurality of regions of the at leastone surface are discontinuously connected to each other at the boundarypositions, respectively, or are not completely continuously connected toeach other at the boundary positions, respectively.

According to another aspect of the invention, there is provided anoptical system which includes an objective lens having a front surfaceand a rear surface. At least one surface of the front and rear surfacesincludes a plurality of regions having different shapes. The objectivelens is formed by injection molding using a mold configured such thatportions corresponding to boundary positions of adjacent ones of theplurality of regions are processed to be continuous surfaces,respectively, by using an R-bite.

With this configuration, it becomes possible to sufficiently suppressthe coma for each of the plurality of types of optical discs. Suchprocess for making the boundary position the continuous surface may beperformed if design shapes of the plurality of regions of the at leastone surface are discontinuously connected to each other at the boundarypositions, respectively, or are not completely continuously connected toeach other at the boundary positions, respectively.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

FIG. 1A shows a configuration of an optical system including anobjective lens according to a first embodiment of the invention when afirst optical disc having higher recording density is used;

FIG. 1B shows of a configuration of the optical system including theobjective lens according to the first embodiment of the invention when asecond optical disc having lower recording density is used;

FIG. 2A is an enlarged view of the objective lens;

FIG. 2B is a cross section of a first surface of the objective lens;

FIG. 2C is a cross section of a second surface of the objective lens;

FIG. 3A is a graph illustrating a wavefront aberration of an on-axis raycaused in the optical system according to a first example when the firstoptical disc is used;

FIG. 3B is a graph illustrating a wavefront aberration of an off-axisray caused in the optical system according to the first example when thefirst optical disc is used;

FIG. 4A is a graph illustrating a wavefront aberration of an on-axis raycaused in the optical system according to the first example when thesecond optical disc is used;

FIG. 4B is a graph illustrating a wavefront aberration of an off-axisray caused in the optical system according to the first example when thesecond optical disc is used;

FIG. 5A is a graph illustrating a wavefront aberration of an on-axis raycaused in the optical system according to a first comparative examplewhen the first optical disc is used;

FIG. 5B is a graph illustrating a wavefront aberration of an off-axisray caused in the optical system according to the first comparativeexample when the first optical disc is used;

FIG. 6A is a graph illustrating a wavefront aberration of an on-axis raycaused in the optical system according to the first comparative examplewhen the second optical disc is used;

FIG. 6B is a graph illustrating a wavefront aberration of an off-axisray caused in the optical system according to the first comparativeexample when the second optical disc is used;

FIG. 7 is a graph illustrating a relationship between the wavefrontaberration of the first example and the image height when the firstoptical disc is used;

FIG. 8 is a graph illustrating a relationship between the wavefrontaberration of the first example and the image height when the secondoptical disc is used;

FIG. 9 is a graph illustrating a relationship between the wavefrontaberration of the first comparative example and the image height whenthe first optical disc is used;

FIG. 10 is a graph illustrating a relationship between the wavefrontaberration of the first comparative example and the image height whenthe second optical disc is used;

FIG. 11A is a graph illustrating a wavefront aberration of an on-axisray caused in the optical system according to a second example when thefirst optical disc is used;

FIG. 11B is a graph illustrating a wavefront aberration of an off-axisray caused in the optical system according to the second example whenthe first optical disc is used;

FIG. 12A is a graph illustrating a wavefront aberration of an on-axisray caused in the optical system according to the second example whenthe second optical disc is used;

FIG. 12B is a graph illustrating a wavefront aberration of an off-axisray caused in the optical system according to the second example whenthe second optical disc is used;

FIG. 13A is a graph illustrating a wavefront aberration of an on-axisray caused in the optical system according to a second comparativeexample when the first optical disc is used;

FIG. 13B is a graph illustrating a wavefront aberration of an off-axisray caused in the optical system according to the second comparativeexample when the first optical disc is used;

FIG. 14A is a graph illustrating a wavefront aberration of an on-axisray caused in the optical system according to the second comparativeexample when the second optical disc is used;

FIG. 14B is a graph illustrating a wavefront aberration of an off-axisray caused in the optical system according to the second comparativeexample when the second optical disc is used;

FIG. 15 is a graph illustrating a relationship between the wavefrontaberration of the second example and the image height when the firstoptical disc is used;

FIG. 16 is a graph illustrating a relationship between the wavefrontaberration of the second example and the image height when the secondoptical disc is used;

FIG. 17 is a graph illustrating a relationship between the wavefrontaberration of the second comparative example and the image height whenthe first optical disc is used;

FIG. 18 is a graph illustrating a relationship between the wavefrontaberration of the second comparative example and the image height whenthe second optical disc is used;

FIG. 19 is a cross section of the objective lens according to the firstembodiment;

FIG. 20 is a cross section of a second surface of an objective lensaccording to a second embodiment;

FIG. 21A shows a configuration of an optical system according to thesecond embodiment when the first optical disc is used;

FIG. 21B shows a configuration of the optical system according to thesecond embodiment when the second optical disc is used;

FIG. 22A is an enlarged view of the objective lens according to thesecond embodiment;

FIG. 22B is a cross section of a first surface of the objective lensshown in FIG. 22A;

FIG. 22C is a cross section of a second surface of the objective lensshown in FIG. 22A;

FIG. 23A is a graph illustrating a wavefront aberration of an on-axisray caused in the optical system according to a third example when thefirst optical disc is used;

FIG. 23B is a graph illustrating a wavefront aberration of an off-axisray caused in the optical system according to the third example when thefirst optical disc is used;

FIG. 24A is a graph illustrating a wavefront aberration of an on-axisray caused in the optical system according to the third example when thesecond optical disc is used;

FIG. 24B is a graph illustrating a wavefront aberration of an off-axisray caused in the optical system according to the third example when thesecond optical disc is used;

FIG. 25A is a graph illustrating a wavefront aberration of an on-axisray caused in the optical system according to a third comparativeexample when the first optical disc is used;

FIG. 25B is a graph illustrating a wavefront aberration of an off-axisray caused in the optical system according to the third comparativeexample when the first optical disc is used;

FIG. 26A is a graph illustrating a wavefront aberration of an on-axisray caused in the optical system according to the third comparativeexample when the second optical disc is used;

FIG. 26B is a graph illustrating a wavefront aberration of an off-axisray caused in the optical system according to the third comparativeexample when the second optical disc is used;

FIG. 27 is a graph illustrating a relationship between the wavefrontaberration of the third example and the image height when the firstoptical disc is used;

FIG. 28 is a graph illustrating a relationship between the wavefrontaberration of the third example and the image height when the secondoptical disc is used;

FIG. 29 is a graph illustrating a relationship between the wavefrontaberration of the third comparative example and the image height whenthe first optical disc is used;

FIG. 30 is a graph illustrating a relationship between the wavefrontaberration of the third comparative example and the image height whenthe second optical disc is used;

FIG. 31A shows a configuration of an optical system according to afourth example of the second embodiment when the first optical disc isused;

FIG. 31B shows a configuration of the optical system according to thefourth example of the second embodiment when the second optical disc isused;

FIG. 32A is a graph illustrating a wavefront aberration of an on-axisray caused in the optical system according to the fourth example whenthe first optical disc is used;

FIG. 32B is a graph illustrating a wavefront aberration of an off-axisray caused in the optical system according to the fourth example whenthe first optical disc is used;

FIG. 33A is a graph illustrating a wavefront aberration of an on-axisray caused in the optical system according to the fourth example whenthe second optical disc is used;

FIG. 33B is a graph illustrating a wavefront aberration of an off-axisray caused in the optical system according to the fourth example whenthe second optical disc is used;

FIG. 34A is a graph illustrating a wavefront aberration of an on-axisray caused in the optical system according to a fourth comparativeexample when the first optical disc is used;

FIG. 34B is a graph illustrating a wavefront aberration of an off-axisray caused in the optical system according to the fourth comparativeexample when the first optical disc is used;

FIG. 35A is a graph illustrating a wavefront aberration of an on-axisray caused in the optical system according to the fourth comparativeexample when the second optical disc is used;

FIG. 35B is a graph illustrating a wavefront aberration of an off-axisray caused in the optical system according to the fourth comparativeexample when the second optical disc is used;

FIG. 36 is a graph illustrating a relationship between the wavefrontaberration of the fourth example and the image height when the firstoptical disc is used;

FIG. 37 is a graph illustrating a relationship between the wavefrontaberration of the fourth example and the image height when the secondoptical disc is used;

FIG. 38 is a graph illustrating a relationship between the wavefrontaberration of the fourth comparative example and the image height whenthe first optical disc is used;

FIG. 39 is a graph illustrating a relationship between the wavefrontaberration of the fourth comparative example and the image height whenthe second optical disc is used;

FIG. 40A is a graph illustrating a wavefront aberration of an on-axisray caused in the optical system according to a fifth example when thefirst optical disc is used;

FIG. 40B is a graph illustrating a wavefront aberration of an off-axisray caused in the optical system according to the fifth example when thefirst optical disc is used;

FIG. 41A is a graph illustrating a wavefront aberration of an on-axisray caused in the optical system according to the fifth example when thesecond optical disc is used;

FIG. 41B is a graph illustrating a wavefront aberration of an off-axisray caused in the optical system according to the fifth example when thesecond optical disc is used;

FIG. 42 is a graph illustrating a relationship between the wavefrontaberration of the fifth example and the image height when the firstoptical disc is used; and

FIG. 43 is a graph illustrating a relationship between the wavefrontaberration of the fifth example and the image height when the secondoptical disc is used.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Hereinafter, embodiments according to the invention are described withreference to the accompanying drawings.

First Embodiment

FIGS. 1A and 1B show an objective lens 10 according to a firstembodiment of the invention. The objective lens 10 is used for recordingdata to and/or reproducing data from a plurality of types of opticaldiscs. The objective lens 10 is employed in an optical disc drive (notshown) which supports the plurality of types of the optical discs. FIG.1A shows an optical path in an optical system of the optical disc drivewhen an optical disc 20A having a higher recording density is used. FIG.1B shows an optical path in the optical system of the optical disc drivewhen an optical disc 20B having a recording density lower than that ofthe optical disc 20A is used.

The optical disc 20A is, for example, a DVD (digital versatile disc)having a cover layer whose thickness is thinner than that of a CD(compact disc). The optical disc 20B is, for example, the CD having arelatively thick cover layer. The optical disc 20A or 20B is placed on aturn table (not shown) in the optical disc drive when the recordingand/or reproducing operation is performed.

When recording and/or reproducing operation for the optical disc 20A isperformed a laser beam having a wavelength of 657 nm (hereafter,referred to as a first laser beam) is emitted from a light source (notshown) so as to form a relatively small beam spot on a data recordinglayer of the optical disc 20A. When recording and/or reproducingoperation for the optical disc 20B is performed a laser beam (hereafter,referred to as a second laser beam) having a wavelength longer than thatof the first laser beam is emitted from the light source so as to form arelatively large beam spot on a data recording layer of the optical disc20B.

As shown in FIGS. 1A and 1B, the optical system is configured such thata collimated beam (first or second laser beam) is incident on theobjective lens 10 for each of the optical disc 20A and 20B. The opticalsystem has collimator lenses which convert the first and second laserbeams emitted by the light source to the collimated beams, respectively.The collimated first laser beam is converged by the objective lens 10onto the data recording layer of the optical disc 20A. The collimatedsecond laser beam is converged by the objective lens 10 onto the datarecording layer of the optical disc 20B.

Since the collimated beam is incident on the objective lens 10,aberrations are not caused when the objective lens 10 is shiftedvertically in FIG. 1A or 1B from a reference axis of the optical systemby tracking operation.

The objective lens 10 is a biconvex plastic single lens having a firstsurface 10 a (front surface) located on a light source side and a secondsurface 10 b (rear surface) located on an optical disc side. Both of thefirst and second surfaces 10 a and 10 b are aspherical surfaces. Sinceas described above the thicknesses of the cover layers of the opticaldiscs 20A and 20B are different from each other, a coma and a sphericalaberration change depending on the type of the optical disc being used.To sufficiently suppress such aberrations, the objective lens 10 isconfigured as follows.

FIG. 2A is an enlarged view of the objective lens 10. FIG. 2B is a crosssection of the first surface 10 a of the objective lens 10 illustratinga section E1 of FIG. 2A. FIG. 2C is a cross section of the secondsurface 10 b of the objective lens 10 illustrating a section E2 of FIG.2B.

As shown in FIG. 2B, the first surface 10 a has an inner region 11 aincluding an optical axis AX of the objective lens 10 and an outerregion 12 a surrounding the inner region 11 a. The outer region 12 aextends from a boundary between the inner region 11 a and the outerregion 12 a to the outermost portion of the objective lens 10. The innerregion 11 a and the outer region 12 a have different shapes.

Also, as shown in FIG. 2C, the second surface 10 b has an inner region11 b including the optical axis AX and an outer region 12 b surroundingthe inner region 11 b. The inner region 11 b and the outer region 12 bhave different shapes. The outer region 12 b on the second surface 10 bis defined as a region through which the beam passed through the outerregion 12 a on the first surface 10 a passes.

The inner regions 11 a and 11 b are regions for attaining the NArequired to obtain a beam spot diameter suitable for recording data toand/or reproducing data from the optical disc 20B.

Since the recording density of the optical disc 20A is higher than thatof the optical disc 20B, a beam spot having a diameter smaller than thatfor the optical disc 20B is required to record data to and/or toreproduce data from the optical disc 20A. In this embodiment, the outerregions 12 a and 12 b are used for attaining an NA larger than that forthe optical disc 20B and thereby forming a smaller beam spot on the datarecording layer of the optical disc 20A. Further, in this embodiment theouter regions 12 a and 12 b are configured not to converge the secondlaser beam.

Each of the first and second surfaces 10 a and 10 b of the objectivelens 10 is divided into two regions (the inter and outer regions) havingdifferent shapes. Therefore, the degree of freedom of a lens designincreases, which enables to configure the objective lens 10 so that acoma caused when the beam is incident on the objective lens 10 obliquelywith respect to the optical axis AX is sufficiently suppressed for eachof the optical discs 20A and 20B.

The detailed configuration of the objective lens 10 for correcting thecoma caused when the beam is incident on the objective lens 10 obliquelywith respect to the optical axis of the objective lens 10 is as follows.

Considering that an extension 32 a of the outer region 12 a, extendingfrom the outer region 12 a toward the optical axis and indicated by achain line in FIG. 2B, the inner region 11 a is configured such that theamount of coma caused when the second laser beam is incident on theinner region 11 a obliquely (at a first angle) with respect to theoptical axis AX is less than the amount of coma caused when the secondlaser beam is incident on the extension 32 a obliquely (at the firstangle) with respect to the optical axis AX.

Similarly, considering that an extension 32 b of the outer region 12 b,extending from the outer region 12 b toward the optical axis andindicated by a chain line in FIG. 2C, the inner region 11 b isconfigured such that the amount of coma caused when the second laserbeam is incident on the inner region 11 b obliquely (at a first angle)with respect to the optical axis AX is less than the amount of comacaused when the second laser beam is incident on the extension 32 bobliquely (at the first angle) with respect to the optical axis AX. Thatis, when the second laser beam is incident, obliquely with respect tothe optical axis AX, on the first (second) surface 10 a (10 b) of theobjective lens 10 at the boundary position P (see FIG. 19) between theinner region and the outer region, the amount of coma caused by theinner region is less than the amount of coma caused by the outer region.

In other words, the inner regions 11 a and 11 b correct the coma, causedwhen the optical disc 20B is used, more sufficiently than the comacaused when the optical disc 20A is used.

The outer regions 12 a and 12 b are configured to correct the comacaused when the optical disc 20A is used.

As described above, the objective lens 10 is configured to suppress thecoma caused when the optical disc 20A is used as well as the coma causedwhen the optical disc 20B is used. Consequently, beam spots suitable forrecording data to and/or reproducing data from the optical discs 20A and20B are formed on the data recording layers of the optical discs 20A and20B, respectively.

To explain the configuration of the objective lens 10 more specifically,an inclination θ of a lens surface is defined as indicated in FIG. 19.FIG. 19 is a cross section of the objective lens 10 including theoptical axis AX. In FIG. 19, symbols have the following meanings.

“P” is a boundary position between the inner region (11 a,11 b) and theouter region (12 a,12 b).“LA” represents the shape of the inner region (11 a,11 b).“LAA” represents an extension of the inner region extended from theboundary position P toward the outer region.“LB” represents the shape of the outer region (12 a,12 b).“LBB” represents an extension of the outer region extended from theboundary position P toward the inner region.“PL” represents a line normal to the lens surface (first surface 10 a orthe second surface 10 b) at the boundary position P.“θ” is the inclination of a lens surface. “θ” is an angle which isformed by a line normal to the lens surface with respect to the opticalaxis AX.Numerical subscripts “1” and “2” of each symbol represent the firstsurface 10 a and the second surface 10 b, respectively.

The inclination of the inner region at the boundary position means thatan angle formed by a line (PL) normal to the lens surface defined by theLA and LAA at the boundary position P with respect to the optical axisAX. The inclination of the outer region at the boundary position meansthat an angle formed by a line (PL) normal to the lens surface definedby the LB and LBB at the boundary position P with respect to the opticalaxis AX.

The inclination θ has a plus sign when it is measured in a clockwisedirection with respect to the optical axis AX, and has a minus sign whenit is measured in a counterclockwise direction with respect to theoptical axis AX.

The words “a coma caused at the boundary position” as used herein meansthat a coma caused at a point nearest to the boundary position on theinner region or on the outer region on one of the first surface and thesecond surface of the objective lens.

The second surface 10 b of the objective lens 10 is configured such thatat the boundary position P₂ the inclination θ₂ of the outer region 12 bis smaller than the inclination θ₂ of the inner region 11 b.

More specifically, the objective lens 10 is configured such that, at theboundary position Pb between the inner region 11 b and the outer regionof the second surface 10 b, the inclination θ_(2A) [degree] of the innerregion 11 b of the second surface 10 b and the inclination θ_(2B)[degree] of the outer region 12 b of the second surface 10 b satisfy thefollowing condition (1).

−2.5<θ_(2B)−θ_(2A)<0.0  (1)

When the θ_(2B)−θ_(2A) gets larger than the upper limit of the condition(1), the effect of correction of the coma decreases for each of theoptical discs 20A and 20B. When the θ_(2B)−θ_(2A) gets lower than thelower limit of the condition (1), the coma is caused particularly whenthe optical disc 20A is used.

In addition to satisfying the condition (1), the objective lens 10 maybe configured such that the inclination θ_(1A) [degree] of the innerregion 11 a and the inclination θ_(1B) [degree] of the outer region 12 aof the first surface 10 a satisfy the following condition (2).

−1.2<θ_(1B)−θ_(1A)<0.0  (2)

When θ_(1B)−θ_(1A) gets larger than the upper limit of the condition(2), the effect of correction of the coma decreases for each of theoptical discs 20A and 20B. When the θ_(1B)−θ_(1A) gets lower than thelower limit of the condition (2), the coma is caused particularly whenthe optical disc 20A is used.

As shown in FIG. 2B, the objective lens 10 may be configured to have adiffracting structure of the first surface 10 a. When the objective lens10 is configured to have the diffracting structure on the first surface10 a, the diffracting structure formed within the inner region 11 a andthe diffracting structure formed within the outer region 12 a havestructures different from each other.

The diffracting structure formed within the inner region 11 a isconfigured such that the first and second laser beams are suitablyconverged onto the data recording layers of the optical discs 20A and20B, respectively. The diffracting structure formed within the outerregion 12 a is configured to suitably converge the first laser beam ontothe data recording layer of the optical disc 20A and to diffuse thesecond laser beam incident thereon (i.e., the outer region 12 a does notcontribute to the formation of the beam spot for the optical disc 20B).

The diffracting structure formed within the outer region 12 a isconfigured such that a wavefront of the first laser beam passed throughthe outer region 12 a is continuously connected to a wavefront of thefirst laser beam passed through the inner region 11 a.

With the above mentioned configuration, a portion of the second laserbeam passed through the inner region 11 a is suitably converged by theobjective lens 10 onto the data recording layer of the optical disc 20B.Consequently, the beam spot suitably for recording data to and/orreproducing data from the optical disc 20B is formed on the datarecording layer of the optical disc 20B. The first laser beam passedthrough the objective lens 10 forms the beam spot, suitable forrecording data to and/or reproducing data from the optical disc 20A, onthe data recording layer of the optical disc 20A.

Although, in the above mentioned first embodiment, the collimated beamis used for each of the optical discs 20A and 20B, the optical systemmay be configured such that a beam other than the collimated beam isincident on the objective lens 10 while the optical system satisfying acondition where magnifications for both of the optical discs 20A and 20Bare the same.

Hereafter, two concrete examples according to the first embodiment ofthe invention will be described. In the following examples, thethicknesses of the cover layers of the optical discs 20A and 20B are 0.6mm and 1.2 mm, respectively.

First Example

Performance specifications of the objective lens 10 according to a firstexample are shown in Table 1.

TABLE 1 First laser Second beam laser beam Design 657 nm 790 nmwavelength Focal length f 3.360 3.384 NA 0.600 0.466 magnification 0.0000.000

In Table 1 (and in the following similar Tables), the design wavelengthis a wavelength suitable for the recordation/reproduction of the opticaldisc, f represents a focal length (unit: mm) of the objective lens 10.NA represents the numerical aperture. In Table 1, the performancespecifications are indicated with regard to each of the first laser beam(the optical disc 20A) and the second laser beam (the optical disc 20B).

Table 2 shows a numerical configuration of the optical system of theoptical disc drive including the objective lens 10 when the optical disc20A is used. Table 3 shows a numerical configuration of the opticalsystem of the optical disc drive including the objective lens 10 whenthe optical disc 20B is used.

TABLE 2 Surface n n Number r d (657 nm) (790 nm) #0 ∞ #1(h ≦ 1.58) 2.1012.21 1.54056 1.53653 #1(1.58 ≦ h) 2.112 #2(h ≦ 1.14) −8.459 1.74 #2(1.14≦ h) −8.450 #3 ∞ 0.60 1.57982 1.57307 #4 ∞ —

TABLE 3 Surface n n Number r d (657 nm) (790 nm) #0 ∞ #1(h ≦ 1.58) 2.1012.21 1.54056 1.53653 #1(1.58 ≦ h) 2.112 #2(h ≦ 1.14) −8.459 1.38 #2(1.14≦ h) −8.450 #3 ∞ 1.20 1.57982 1.57307 #4 ∞ —

In Tables 2 and 3, “surface number” represents a surface number of eachsurface of optical components in the optical system. In Tables 2 and 3,a surface #0 represents the light source, and surfaces #1 and #2represent the first surface 10 a and the second surface 10 b of theobjective lens 10, respectively. In Table 2, surfaces #3 and #4represent the cover layer and the data recording layer of the opticaldisc 20A, respectively. In Table 3, surfaces #3 and #4 represent thecover layer and the data recording layer of the optical disc 20B,respectively.

In Tables 2 and 3 (and in the following similar Tables), “r” representsa radius of curvature (unit: mm) of each lens surface on the opticalaxis. “d” represents a thickness of a lens or a distance (unit: mm) froma lens surface to a next lens surface. “n” represents a refractive indexwhich is indicated for each of wavelengths of the first and second laserbeams.

As shown in Tables 2 and 3, the first surface 10 a of the objective lens10 is divided into the inner region 11 a and the outer region 12 a,which are defined by the height h (mm) from the optical axis (AX) asfollows.

inner region 11 a: h≦1.58outer region 12 a: 1.58≦h

Similarly, as shown in Tables 2 and 3, the second surface 10 b of theobjective lens 10 is divided into the inner region 11 b and the outerregion 12 b, which are defined by the height h (mm) from the opticalaxis (AX) as follows.

inner region 11 b: h≦1.14outer region 12 b: 1.14≦h

The first surface 10 a (#1) and the second surface 10 b (#2) of theobjective lens 10 are aspherical surfaces. The inner region 11 a and theouter region 12 a of the first surface 10 a are configured to bedifferent aspherical shapes. Also, the inner region 11 b and the outerregion 12 b of the second surface 10 b are configured to be differentaspherical shapes.

The aspherical surface is expressed by a following equation:

${X(h)} = {\frac{{ch}^{2}}{1 + \sqrt{1 - {\left( {1 + K} \right)c^{2}h^{2}}}} + {A_{4}h^{4}} + {A_{6}h^{6}} + {A_{10}h^{10}} + {A_{12}h^{12}}}$

where, X(h) represents a SAG amount which is a distance between a pointon the aspherical surface at a height of h from the optical axis and aplane tangential to the aspherical surface at the optical axis, symbol crepresents curvature (1/r) on the optical axis, K is a conicalcoefficient, and A₄, A₆, A₈, A₁₀ and A₁₂ are aspherical coefficients offourth, sixth, eighth, tenth and twelfth orders, respectively.

Table 4 shows the conical coefficients and aspherical coefficients ofthe first surface 10 a (#1) and the second surface 10 b (#2) of theobjective lens 10 according to the first example.

TABLE 4 Surface No. #1 #1 #2 #2 (h ≦ 1.58) (1.58 ≦ h) (h ≦ 1.14) (1.14 ≦h) K −0.5000 −0.5000 0.0000 0.0000 A4 −2.3550E−03 −1.1290E−03 1.2690E−021.4360E−02 A6 −1.9490E−04 −8.6690E−05 −8.1610E−04 −2.0470E−03 A88.8930E−06 −8.9330E−05 8.9730E−05 −3.6610E−04 A10 3.3930E−06 2.0383E−05−1.7630E−05 1.2890E−04 A12 6.6810E−08 −5.6202E−06 1.0025E−06 −1.1338E−05

In Table 4 (and in the following similar Tables), a notation symbol Eindicates that 10 is used as a radix and a right side value of E is usedas an exponent.

The first surface 10 a of the objective lens 10 has a diffractingstructure. The diffracting structure is expressed by an optical pathdifference function Φ(h):

Φ(h)=(P ₂ h ² +P ₄ h ⁴ +P ₆ h ⁶+ . . . )mλ

where P₂, P₄ and P₆ are coefficients of second, fourth and sixth orders,h represents a height from the optical axis, m represents a diffractionorder, and λ represents a working wavelength. The optical pathdifference Φ(h) indicates a difference of an optical path length of ahypothetical ray of light which does not pass through the diffractingstructure and an optical path length of a ray of light which isdiffracted by the diffracting structure, at the height h from theoptical axis. In other words, the optical path difference Φ(h)represents the additional optical path length of each ray of light whichis diffracted by the diffracting structure. “m” represents thediffraction order used for the recording and/or reproducing operation.In this example, m is 1.

Table 5 shows values of the coefficients of the optical path differencefunction Φ(h) applied to the diffracting structure formed within theinner region 11 a and the outer region 12 a of the first surface 10 a(#1).

TABLE 5 #1 #1 coefficient (h ≦ 1.58) (1.58 ≦ h) P2 0.0000E+00−9.9918E−01 P4 −1.6568E+00 −8.7030E−01 P6 −1.3534E−01 −1.5720E−01 P80.0000E+00 0.0000E+00 P10 0.0000E+00 0.0000E+00 P12 0.0000E+000.0000E+00

In this example, at the boundary position Pb between the inner region 11b and the outer region 12 b of the second surface 10 b, the inclinationθ_(2A) [degree] of the inner region 11 b of the second surface 10 b is−3.94 [degree], and the inclination θ_(2B) [degree] of the outer region12 b of the second surface 10 b is −4.48 [degree]. In this case,θ_(2B)−θ_(2A)=−0.52 [degree]. Therefore, in this example, the condition(1) is satisfied. In this example, the condition (2) is also satisfied.

FIGS. 3A and 3B are graphs illustrating wavefront aberrations caused inthe optical system according to the first example when the optical disc20A is used. FIGS. 4A and 4B are graphs illustrating wavefrontaberrations caused in the optical system according to the first examplewhen the optical disc 20B is used. Each of FIGS. 3A and 4A shows thewavefront aberration regarding an on-axis ray, and each of FIGS. 3B and4B shows the wavefront aberration regarding an off-axis ray (at an imageheight of 0.06 mm).

An objective lens according to a first comparative example is consideredas follows for comparing with the objective lens 10 according the firstexample. The objective lens according to the first comparative examplehas substantially the same configuration as that of the objective lens10 of the first example, but a second surface (an optical disc sidesurface) thereof is configured to be a single continuous surface. Thatis, the second surface of the objective lens according to the firstcomparative example is not divided into an inner region and an outerregion.

FIGS. 5A and 5B are graphs illustrating wavefront aberrations caused inan optical system having an objective lens according to the firstcomparative example when the optical disc 20A is used. FIGS. 6A and 6Bare graphs illustrating wavefront aberrations caused in the opticalsystem according to the first comparative example when the optical disc20B is used. Each of FIGS. 5A and 6A shows the wavefront aberrationregarding an on-axis ray, and each of FIGS. 5B and 6B shows thewavefront aberration regarding an off-axis ray (at an image height of0.06 mm).

FIGS. 7 and 8 show the wavefront aberrations caused in the first examplewhen the optical discs 20A and 20B are used, respectively. Morespecifically, FIG. 7 is a graph illustrating a relationship between thewavefront aberration rms[λ] and the image height [mm] when the opticaldisc 20A is used. FIG. 8 is a graph illustrating a relationship betweenthe wavefront aberration rms[λ] and the image height [mm] when theoptical disc 20B is used.

FIGS. 9 and 10 show the wavefront aberrations caused in the opticalsystem including the objective lens according to the first comparativeexample when the optical discs 20A and 20B are used, respectively. FIG.9 is a graph illustrating a relationship between the wavefrontaberration rms[λ] and the image height [mm] when the optical disc 20A isused in the optical system including the objective lens according to thefirst comparative example. FIG. 10 is a graph illustrating arelationship between the wavefront aberration rms[λ] and the imageheight [mm] when the optical disc 20B is used in the optical systemincluding the objective lens according to the first comparative example.

In each of FIGS. 7-10 (and in the following similar drawings), a curveindicated by a “coma 3” represents the coma of the third order, a curveindicated by an “as 3” represents an astigmatism of the third order, anda curve indicated by a “coma 5” represents the coma of the fifth order.

The amount of the wavefront aberration caused when the optical disc 20Ais used is analyzed as follows by making a comparison between FIG. 7 andFIG. 9. The coma of the third order caused in the case of the firstexample is reduced to a level substantially equal to the coma of thethird order caused in the case of the first comparative example.

The amount of the wavefront aberration caused when the optical disc 20Bis used is analyzed as follows by making a comparison between FIG. 8 andFIG. 10. The coma of the third order caused in the case of the firstexample is reduced more sufficiently than the coma of the third ordercaused in the case of the first comparative example. That is, the amountof the wavefront aberration caused in the case of the first example isreduced more sufficiently than the wavefront aberration caused in thecase of the first comparative example.

Therefore, according to the first example, the coma of the third orderis sufficiently suppressed for both of the optical discs 20A and 20B.Consequently, beam spots suitable for recording data to and/orreproducing data from the optical discs 20A and 20B can be formed on thedata recording layers of the optical discs 20A and 20 b, respectively.

Although the coma of the fifth order caused in the case of the firstexample is slightly larger than the coma of the fifth order caused inthe cased of the first comparative example, the amount of the coma ofthe fifth order caused in the case of the first example does not affectthe formation of the suitable beam spots for the optical discs 20A and20B.

Second Example

Performance specifications of the objective lens 10 according to asecond example are shown in Table 6.

TABLE 6 First laser Second laser beam beam Design 657 nm 790 nmwavelength Focal length f 3.360 3.384 NA 0.600 0.467 magnification 0.0000.000

Table 7 shows a numerical configuration of an optical system of anoptical disc drive including the objective lens 10 according to thesecond example when the optical disc 20A is used. Also, Table 8 shows anumerical configuration of the optical system of the optical disc driveincluding the objective lens 10 according to the second example when theoptical disc 20B is used.

TABLE 7 Surface n n Number r d (657 nm) (790 nm) #0 ∞ #1(h ≦ 1.58)2.1090 2.21 1.54056 1.53653 #1(1.58 ≦ h) 2.1080 #2(h ≦ 1.15) −8.27601.75 #2(1.15 ≦ h) −8.3147 #3 ∞ 0.60 1.57982 1.57307 #4 ∞ —

TABLE 8 Surface n n Number r d (657 nm) (790 nm) #0 ∞ #1(h ≦ 1.58)2.1090 2.21 1.54056 1.53653 #1(1.58 ≦ h) 2.1080 #2(h ≦ 1.15) −8.27601.40 #2(1.15 ≦ h) −8.3147 #3 ∞ 1.20 1.57982 1.57307 #4 ∞ —

In Tables 7 and 8, “surface number” represents a surface number of eachsurface of optical components in the optical system. In Tables 7 and 8,a surface #0 represents the light source, and surfaces #1 and #2represent the first surface 10 a and the second surface 10 b of theobjective lens 10, respectively. In Table 7, surfaces #3 and #4represent the cover layer and the data recording layer of the opticaldisc 20A, respectively. In Table 8, surfaces #3 and #4 represent thecover layer and the data recording layer of the optical disc 20B,respectively.

As shown in Tables 7 and 8, the first surface 10 a of the objective lens10 is divided into the inner region 11 a and the outer region 12 a,which are defined by the height h (mm) from the optical axis (AX) asfollows.

inner region 11 a: h≦1.58outer region 12 a: 1.58≦h

Similarly, as shown in Tables 7 and 8, the second surface 10 b of theobjective lens 10 is divided into the inner region 11 b and the outerregion 12 b, which are defined by the height h (mm) from the opticalaxis (AX) as follows.

inner region 11 b: h≦1.15outer region 12 b: 1.15≦h

The first surface 10 a (#1) and the second surface 10 b (#2) of theobjective lens 10 are aspherical surfaces which are defined by the abovementioned equation. Further, the inner region 11 a and the outer region12 a of the first surface 10 a are configured to be different asphericalshapes. Also, the inner region 11 b and the outer region 12 b of thesecond surface 10 b are configured to be different aspherical shapes.

Table 9 shows the conical coefficients and aspherical coefficients ofthe first surface 10 a (#1) and the second surface 10 b (#2) of theobjective lens 10 according to the second example.

TABLE 9 Surface No. #1 #1 #2 #2 (h ≦ 1.58) (1.58 ≦ h1) (h ≦ 1.15) (1.15≦ h) K −0.5000 −0.5000 0.0000 0.0000 A4 −2.4820E−04 −2.4410E−041.3570E−02 1.3530E−02 A6 7.2160E−05 2.7270E−05 −9.8090E−04 −1.7710E−03A8 2.3780E−05 −5.2640E−06 2.0850E−05 −6.4380E−05 A10 2.8940E−061.5870E−06 −7.9860E−06 5.4570E−05 A12 7.7610E−08 −1.2972E−06 1.4650E−06−5.3289E−06

In this example, at the boundary position Pa between the inner region 11a and the outer region 12 a of the first surface 10 a, the inclinationθ_(1A) [degree] of the inner region 11 a of the first surface 10 a is41.57 [degree], and the inclination θ_(1B) [degree] of the outer region12 a of the first surface 10 a is 41.22 [degree]. In this case,θ_(1B)−θ_(1A)=−0.35 [degree]. Therefore, the condition (2) is satisfied.

In this example, at the boundary position Pb between the inner region 11b and the outer region 12 b of the second surface 10 b, the inclinationθ_(2A) [degree] of the inner region 11 b of the second surface 10 b is−7.75 [degree], and the inclination θ_(2B) [degree] of the outer region12 b of the second surface 10 b is −7.77 [degree]. In this case,θ_(2B)−θ_(2A)=−0.02 [degree]. Therefore, the condition (1) is satisfied.

FIGS. 11A and 11B are graphs illustrating wavefront aberrations causedin the optical system according to the second example when the opticaldisc 20A is used. FIGS. 12A and 12B are graphs illustrating wavefrontaberrations caused in the optical system according to the second examplewhen the optical disc 20B is used. Each of FIGS. 11A and 12A shows thewavefront aberration regarding an on-axis ray, and each of FIGS. 11B and12B shows the wavefront aberration regarding an off-axis ray (at animage height of 0.06 mm).

An objective lens according to a second comparative example isconsidered as follows for comparing with the objective lens 10 accordingthe second example. The objective lens according to the secondcomparative example has substantially the same configuration as that ofthe objective lens 10 of the second example, but a second surface (anoptical disc side surface) thereof is configured to be a singlecontinuous surface. That is, the second surface of the objective lensaccording to the second comparative example is not divided into an innerregion and an outer region.

FIGS. 13A and 13B are graphs illustrating wavefront aberrations causedin an optical system having an objective lens according to the secondcomparative example when the optical disc 20A is used. FIGS. 14A and 14Bare graphs illustrating wavefront aberrations caused in the opticalsystem according to the second comparative example when the optical disc20B is used. Each of FIGS. 13A and 14A shows the wavefront aberrationregarding an on-axis ray, and each of FIGS. 13B and 14B shows thewavefront aberration regarding an off-axis ray (at an image height of0.06 mm).

FIGS. 15 and 16 respectively show the wavefront aberrations caused inthe second example when the optical discs 20A and 20B are used. Morespecifically, FIG. 15 is a graph illustrating a relationship between thewavefront aberration rms[λ] and the image height [mm] when the opticaldisc 20A is used. FIG. 16 is a graph illustrating a relationship betweenthe wavefront aberration rms[λ] and the image height [mm] when theoptical disc 20B is used.

FIGS. 17 and 18 respectively show the wavefront aberrations caused, inthe optical system including the objective lens according to the secondcomparative example, when the optical discs 20A and 20B are used. FIG.17 is a graph illustrating a relationship between the wavefrontaberration rms[λ] and the image height [mm] when the optical disc 20A isused in the optical system including the objective lens according to thesecond comparative example. FIG. 18 is a graph illustrating arelationship between the wavefront aberration rms[λ] and the imageheight [mm] when the optical disc 20B is used in the optical systemincluding the objective lens according to the second comparativeexample.

The amount of the wavefront aberration caused when the optical disc 20Ais used is analyzed as follows by making a comparison between FIG. 15and FIG. 17. Although the coma of the third order caused in the case ofthe second example is slightly larger than the coma of the third ordercaused in the case of the second comparative example, the coma of thethird order caused in the second example is sufficiently suppressed.

The amount of the wavefront aberration caused when the optical disc 20Bis used is analyzed as follows by making a comparison between FIG. 16and FIG. 18. The coma of the third order caused in the case of thesecond example is reduced more sufficiently than the coma of the thirdorder caused in the case of the second comparative example.

Therefore, according to the second example, the coma of the third orderis sufficiently suppressed for both of the optical discs 20A and 20B.Consequently, beam spots suitable for recording data to and/orreproducing data from the optical discs 20A and 20B can be formed on thedata recording layers of the optical discs 20A and 20 b, respectively.

Second Embodiment

Hereafter, a second embodiment of the invention will be described. Eachof FIGS. 21A and 21B show an optical system 200 including an objectivelens 210 according to a second embodiment of the invention. Theobjective lens 210 is used for recording data to and/or reproducing datafrom a plurality of types of optical discs. FIG. 21A shows aconfiguration of the optical system 200 when an optical disc 20A isused. That is, the FIG. 21A shows an optical path when the optical disc20A is used. FIG. 21B shows a configuration of the optical system 200when an optical disc 20B is used. That is, the FIG. 21B shows an opticalpath when the optical disc 20A is used.

Similarly to the first embodiment, the optical disc 20A is, for example,the DVD having a cover layer whose thickness is thinner than that of theCD. The optical disc 20B is, for example, the CD having a relativelythick cover layer.

As shown in FIGS. 21A and 21B, the optical system 200 includes a lightsource 30, a branching element 40, a coupling lens 50 and the objectivelens 210. The optical system 200 is mounted on an optical disc driveused for recording data to and/or reproducing data from the plurality oftypes of optical discs. The optical disc 20A or 20B is placed on a turntable (not shown) in the optical disc drive when the recording and/orreproducing operation is performed.

When recording and/or reproducing operation for the optical disc 20A isperformed a laser beam having a relatively short wavelength (hereafter,referred to as a first laser beam) is emitted from the light source 30so as to form a relatively small beam spot on a data recording layer ofthe optical disc 20A. The wavelength of the first laser beam is, forexample, 657 nm. When recording and/or reproducing operation for theoptical disc 20B is performed a laser beam (hereafter, referred to as asecond laser beam) having a wavelength longer than that of the firstlaser beam is emitted from the light source 30 so as to form arelatively large beam spot on a data recording layer of the optical disc20B. The wavelength of the second laser beam is, for example, 790 nm.

The light source 30 has two light emitting portions which emit the firstand second laser beams, respectively. One light emitting portion for thefirst laser beam is located on an optical axis of the objective lens210. The other light emitting portion for the second laser beam slightlyshifts from the optical axis. Therefore, the second laser beam isincident on the objective lens 210 obliquely with respect to the opticalaxis of the objective lens 210.

The first or second laser beam emitted by the light source 30 isincident on the coupling lens 50 via the branching element 40, and adivergence of the laser beam is changed by the coupling lens 50. Thefirst laser beam is converged by the objective lens 210 onto the datarecording layer of the optical disc 20A. The second laser beam isconverged by the objective lens 210 onto the data recording layer of theoptical disc 20B.

Since the collimated beam or a slightly diverging beam is incident onthe objective lens 210, aberrations including a spherical aberration anda coma, caused when the objective lens 210 is shifted vertically in FIG.21A or 21B from a reference axis (indicated by a chain line in FIGS. 21Aand 21B) of the optical system 200 by tracking operation, aresufficiently small.

A returning beam reflected from the data recording layer of the opticaldisc 20A or 20B passes through the objective lens 210 and the couplinglens 50, and then is incident on the branching element 40 again. Thebranching element 40 has, for example, a diffraction element having adiffraction grating on its light source side surface 40 a.

The returning beam incident on the branching element 40 is diffracted bythe diffraction grating on the surface 40 a and is thereby deviated fromthe optical path along which the laser beam emitted by the light source30 proceeds toward the branching element 40. Finally, the returning beamis incident on a photoreceptor (not shown) located in the vicinity ofthe light source 30.

The objective lens 210 is a biconvex plastic single lens having a firstsurface 210 a located on a light source side and a second surface 210 blocated on an optical disc side. Both of the first and second surfaces210 a and 210 b are aspherical surfaces.

Since as described above the thicknesses of the cover layers of theoptical discs 20A and 20B are different from each other, the coma andthe spherical aberration change depending on the type of the opticaldisc being used. To sufficiently suppress such aberrations, theobjective lens 210 is configured as follows.

FIG. 22A is an enlarged view of the objective lens 210. FIG. 22B is across section of the first surface 210 a of the objective lens 210illustrating a section F1 in FIG. 22A. FIG. 2C is a cross section of thesecond surface 210 b of the objective lens 210 illustrating a section F2in FIG. 22A.

As shown in FIG. 22B, the first surface 210 a has an inner region 211 aincluding the optical axis AX of the objective lens 210 and an outerregion 212 a surrounding the inner region 211 a. The outer region 212 aextends from a boundary between the inner region 211 a and the outerregion 212 a to the outermost portion of the objective lens 210. Theinner region 211 a and the outer region 212 a are configured to havedifferent shapes.

Also, as shown in FIG. 22C, the second surface 210 b has an inner region211 b including the optical axis AX and an outer region 212 bsurrounding the inner region 211 b. The inner region 211 b and the outerregion 212 b are configured to have different shapes. The outer region212 b on the second surface 210 b is defined as a region through whichthe beam passed through the outer region 212 a on the first surface 210a passes.

The inner regions 211 a and 211 b are regions for attaining the NArequired to obtain a beam spot diameter suitable for recording data toand/or reproducing data from the optical disc 20B.

Since the recording density of the optical disc 20A is higher than thatof the optical disc 20B, a beam spot having a diameter smaller than thatfor the optical disc 20B is required to record data to and/or toreproduce data from the optical disc 20A. In this embodiment, the outerregions 212 a and 212 b are used for attaining an NA larger than thatfor the optical disc 20B and thereby forming a smaller beam spot on thedata recording layer of the optical disc 20A. Further, in thisembodiment the outer regions 212 a and 212 b are configured not toconverge the second laser beam.

Each of the first and second surfaces 210 a and 210 b of the objectivelens 210 is divided into two regions (the inter and outer regions)having different shapes. Therefore, the degree of freedom of a lensdesign increases, which enables to configure the objective lens 210 sothat the coma caused when the beam is incident on the objective lens 210obliquely with respect to the optical axis AX is sufficiently suppressedfor each of the optical discs 20A and 20B.

The detailed configuration of the objective lens 210 for correcting thecoma caused when the beam is incident on the objective lens 210obliquely with respect to the optical axis AX of the objective lens 210is as follows.

The first surface 210 a is configured such that, when the optical disc20B (the second laser beam) is used, the amount of coma caused by theinner region 211 a at a boundary position Pa between the inner region211 a and the outer region 212 a is less than the amount of coma casedby the outer region 212 a at the boundary position Pa. Similarly, thesecond surface 210 b is configured such that, when the optical disc 20B(the second laser beam) is used, the amount of coma caused by the innerregion 211 b at a boundary position Pb between the inner region 211 band the outer region 212 b is less than the amount of coma cased by theouter region 212 b at the boundary position Pb.

In other words, the inner regions 211 a and 211 b correct the coma,caused when the optical disc 20B is used, more sufficiently than thecoma caused when the optical disc 20A is used.

The outer regions 212 a and 212 b are configured to correct the comacaused when the optical disc 20A is used.

As described above, the objective lens 210 is configured to suppress thecoma caused when the optical disc 20A is used as well as the coma causedwhen the optical disc 20B is used. Consequently, beam spots suitable forrecording data to and/or reproducing data from the optical discs 20A and20B are formed on the data recording layers of the optical discs 20A and20B, respectively.

The configuration of the objective lens 210 is described morespecifically referring to FIG. 19, and hereafter the symbols in FIG. 19are frequently used again to explain the configuration of the objectivelens 210 in detail.

The second surface 210 b of the objective lens 210 is configured suchthat at the boundary position P₂ the inclination θ₂ of the outer region212 b is smaller than the inclination θ₂ of the inner region 211 b.

More specifically, the objective lens 210 is configured such that, atthe boundary position Pb between the inner region 211 b and the outerregion 212 b of the second surface 210 b, the inclination θ_(2A)[degree] of the inner region 211 b of the second surface 210 b and theinclination θ_(2B) [degree] of the outer region 212 b of the secondsurface 210 b satisfy the following condition (4).

−2.5<θ_(2B)−θ_(2A)<−0.05  (4)

Preferably, the first surface 210 a of the objective lens 210 isconfigured such that the inclination θ_(1A) [degree] of the inner region211 a and the inclination θ_(1B) [degree] of the outer region 212 asatisfy the condition (2).

−1.2<θ_(1B)−θ_(1A)<0.0  (2)

As shown in FIG. 22B, the first surface 210 a has a diffractingstructure. The diffracting structure formed within the inner region 211a and the diffracting structure formed within the outer region 212 a aredifferent from each other.

The diffracting structure formed within the inner region 211 a isconfigured such that the first and second laser beams are suitablyconverged onto the data recording layers of the optical discs 20A and20B, respectively. The diffracting structure formed within the outerregion 212 a is configured to suitably converge the first laser beamonto the data recording layer of the optical disc 20A and to diffuse thesecond laser beam incident thereon (i.e., the outer region 212 a doesnot contribute to the formation of the beam spot for the optical disc20B).

The diffracting structure formed within the outer region 212 a isconfigured such that a wavefront of the first laser beam passed throughthe outer region 212 a is continuously connected to a wavefront of thefirst laser beam passed through the inner region 211 a.

With the above mentioned configuration, a portion of the second laserbeam passed through the inner region 211 a is suitably converged by theobjective lens 210 onto the data recording layer of the optical disc20B. Consequently, the beam spot suitable for recording data to and/orreproducing data from the optical disc 20B is formed on the datarecording layer of the optical disc 20B. The first laser beam passedthrough the objective lens 210 forms the beam spot, suitable forrecording data to and/or reproducing data from the optical disc 20A, onthe data recording layer of the optical disc 20A.

In addition to the above mentioned configuration, the objective lens 210according to the second embodiment has an additional feature describedbelow.

Before explaining the additional feature of the objective lens 210, somepossible problems which arise if a lens surface is formed by two regionshaving different shapes are discussed. When the lens surface is formedby the two regions, there may be a case where a step is formed at aboundary between the two regions. The possible problems which may arisewhen such a step is formed at the boundary on the second surface 210 bof the objective lens 210 are that:

(1) the step may be damaged during the grinding process conducted byusing a lens cleaner, and thereby dirt and debris may remain on the lenssurface; or(2) corruption of a shape of the step formed on the lens surface mayoccur due to the faulty transferring in an injection molding process,and thereby loss of the light amount is caused by the corrupted shape ofthe step on the lens surface.These problems become causes of reduction in quality of the objectivelens.

To avoid the above mentioned problems, the second surface 210 b of theobjective lens 210 is configured as follows. FIG. 20 is a partialenlarged view of the objective lens 210 illustrating a cross section ofthe second surface 210 b. In FIG. 20, to portions which are the same asthose shown in FIG. 19, the same symbols are used, and explanationsthereof will not be repeated.

In FIG. 20, symbols have the following meanings. “LC₂” represents ashape of a cross section at the boundary position P₂ when the boundaryposition is smoothed.

The extension LAA may be represented as a surface defined by a surfaceshape of the inner region. Also, the extension LBB may be represented asa surface defined by a surface shape of the outer region. When theextensions LAA and LBB are represented as described above, “X_(A)” and“X_(B)” can be defined as follows.“X_(A)” represents a distance, measured at the boundary position P₂,between the surface LAA₂ defined by the surface shape of the innerregion LA₂ to a plane T1 tangential to the second surface 210 b at theoptical axis AX.“X_(B)” represents a distance, measured at the boundary position P₂,between the surface LBB₂ defined by the surface shape of the outerregion LB₂ to the plane T1 tangential to the second surface 210 b at theoptical axis AX.

More specifically, the surface defined by the surface shape of the innerregion is a surface defined by the equation representing the innerregion, and the surface defined by the surface shape of the outer regionis a surface defined by the equation representing the outer region. Thedistance X_(A) is determined based on a surface obtained by assigning avalue of height h from the optical axis corresponding to the boundaryposition P to the equation of the inner region. The distance X_(B) isdetermined based on a surface obtained by assigning a value of height hfrom the optical axis corresponding to the boundary position P to theequation of the outer region.

The second surface 210 b is configured such that the distance X_(A) atthe boundary position P₂ and the distance X_(B) at the boundary positionP₂ satisfy the following condition (3).

−1.0×10⁻³ <X _(B) −X _(A)<1.0×10⁻³  (3)

By satisfying the condition (3), the height of the step formed on thesecond surface 210 b between the inner region 211 b and the outer region212 b is kept at low level, and thereby the above mentioned problems areeffectively prevented.

Since the first surface 210 a has the diffracting structure, even if astep is formed at the boundary between the inner region 211 a and theouter region 212 a, the step does not cause any problem.

Although, in the above mentioned second embodiment, the collimated beamis used for each of the optical discs 20A and 20B, the optical systemmay be configured such that a beam other than the collimated beam isincident on the objective lens 210 while the optical system 200satisfying a condition where magnifications for both of the opticaldiscs 20A and 20B are the same.

Hereafter, three concrete examples (third, fourth and fifth examples)according to the second embodiment of the invention will be described.In the following examples, the thicknesses of the cover layers of theoptical discs 20A and 20B are 0.6 mm and 1.2 mm, respectively.

Third Example

The configuration of the optical system 200 according to a third examplewhen the optical disc 20A is used is shown in FIG. 21A. Theconfiguration of the optical system 200 according to the third examplewhen the optical disc 20B is used is shown in FIG. 21B. Performancespecifications of the objective lens 210 according to the third exampleare shown in Table 10.

TABLE 10 First laser Second laser beam beam Design 657 nm 790 nmwavelength Focal length f 3.360 3.384 NA 0.600 0.465 magnification 0.0000.000

Table 11 shows a numerical configuration of the optical system 200according to the third example when the optical disc 20A is used. Table12 shows a numerical configuration of the optical system 200 accordingto the third example when the optical disc 20B is used.

TABLE 11 Surface n n Number r d (657 nm) (790 nm) #0 2.36 #1 ∞ 1.501.51383 1.51052 #2 ∞ 17.00 #3 126.470 1.77 1.54056 1.53653 #4 −12.67010.00 #5 (h ≦ 1.57) 2.080 2.21 1.54056 1.53653 #5 (1.57 ≦ h) 2.085 #6 (h≦ 1.12) −8.981 1.73 #6 (1.12 ≦ h) −9.071 #7 ∞ 0.60 1.57982 1.57307 #8 ∞—

TABLE 12 Surface n n Number r d (657 nm) (790 nm) #0 2.36 #1 ∞ 1.501.51383 1.51052 #2 ∞ 17.00 #3 126.470 1.77 1.54056 1.53653 #4 −12.67010.36 #5 (h ≦ 1.57) 2.080 2.21 1.54056 1.53653 #5 (1.57 ≦ h) 2.085 #6 (h≦ 1.12) −8.981 1.37 #6 (1.12 ≦ h) −9.071 #7 ∞ 1.20 1.57982 1.57307 #8 ∞—

In Tables 11 and 12, “surface number” represents a surface number ofeach surface of optical components in the optical system. In Tables 12and 13, a surface #0 represents the light source 30, surfaces #1 and #2represent a light source side surface and an optical disc side surfaceof the branching element 40, respectively, surfaces #3 and #4 representa light source side surface and an optical disc side surface of thecoupling lens 50, respectively, and surfaces #5 and #6 represent thefirst surface 210 a and the second surface 210 b of the objective lens210, respectively. In Table 11, surfaces #7 and #8 represent the coverlayer and the data recording layer of the optical disc 20A,respectively. In Table 12, surfaces #7 and #8 represent the cover layerand the data recording layer of the optical disc 20B, respectively.

As shown in Tables 11 and 12, the first surface 210 a of the objectivelens 210 is divided into the inner region 211 a and the outer region 212a by the boundary position Pa having the height h of 1.57 mm from theoptical axis AX. That is, the inner region 211 a and the outer region212 a are defined as follows.

inner region 211 a: h≦1.57outer region 212 a: 1.57≦h

Similarly, the second surface 210 b of the objective lens 210 is dividedinto the inner region 211 b and the outer region 212 b by the boundaryposition Pb having the height h of 1.12 mm from the optical axis AX.That is, the inner region 211 b and the outer region 212 b are definedas follows.

inner region 211 b: h≦1.12outer region 212 b: 1.12≦h

The optical disc side surface (#4) of the coupling lens 50, the firstsurface 210 a (#5) and the second surface 210 b (#6) of the objectivelens 210 are aspherical surfaces which are defined by the abovementioned equation. The inner region 211 a and the outer region 212 a ofthe first surface 210 a are configured to be different asphericalshapes. Also, the inner region 211 b and the outer region 212 b of thesecond surface 210 b are configured to be different aspherical shapes.

Table 13 shows the conical coefficients and aspherical coefficients ofthe optical disc side surface (#4) of the coupling lens 50, and thefirst surface 210 a (#5) and the second surface 210 b (#6) of theobjective lens 210 according to the third example.

TABLE 13 Surface K A4 A6 #4 0.0000 4.3890E−05 2.0480E−07 #5 (h ≦ 1.57)−0.5000 −2.0210E−03 −2.0980E−04 #5 (1.57 ≦ h) −0.5000 −1.0040E−03−1.8960E−04 #6 (h ≦ 1.12) 0.0000 1.4010E−02 −2.0380E−03 #6 (1.12 ≦ h)0.0000 1.4730E−02 −2.0270E−03 A8 A10 A12 #4 8.2010E−10 0.0000E+000.0000E+00 #5 (h ≦ 1.57) 2.0380E−05 1.6870E−06 3.2222E−06 #5 (1.57 ≦ h)1.8620E−05 −8.4890E−06 4.2320E−07 #6 (h ≦ 1.12) 9.6450E−04 −1.2650E−04−2.7244E−06 #6 (1.12 ≦ h) 3.0190E−06 6.2430E−05 −7.8700E−06

The first surface 210 a of the objective lens 210 has a diffractingstructure expressed by the above mentioned optical path differencefunction Φ(h). In this example, the diffraction order m used for therecording and/or reproducing operation is 1. Table 14 shows values ofthe coefficients of the optical path difference function Φ(h) applied tothe diffracting structure formed within the inner region 211 a and theouter region 212 a of the first surface 210 a.

TABLE 14 surface P2 P4 P6 #5 (h ≦ 1.57) 0.0000E+00 −1.6550E+00−1.3860E−01 #5 (1.57 ≦ h) −6.5260E−01 −9.6980E−01 −1.6230E−01 P8 P10 P12#5 (h ≦ 1.57) 0.0000E+00 0.0000E+00 0.0000E+00 #5 (1.57 ≦ h) 0.0000E+000.0000E+00 0.0000E+00

With regard to the conditions (3) and (4), the objective lens 210according to the third example take values of the distances X_(A) andX_(B), and the inclinations θ_(2A) and θ_(2B) indicated in Table 15. Asshown in Table 15, the objective lens 210 according to the third examplesatisfies the conditions (3) and (4),

TABLE 15 Condition (3) Condition (4) XA −0.0502 θ_(2A) −3.15 XB −0.0501θ_(2B) −3.52 XB − XA 0.0001 θ_(2B) − θ_(2A) −0.37

FIGS. 23A and 23B are graphs illustrating wavefront aberrations causedin the optical system 210 according to the third example when theoptical disc 20A is used. FIGS. 24A and 24B are graphs illustratingwavefront aberrations caused in the optical system 210 according to thethird example when the optical disc 20B is used. Each of FIGS. 23A and24A shows the wavefront aberration regarding an on-axis ray, and each ofFIGS. 23B and 24B shows the wavefront aberration regarding an off-axisray (at an image height of 0.06 mm).

An objective lens according to a third comparative example is consideredas follows for comparing with the objective lens 210 according the thirdexample. The objective lens according to the third comparative examplehas substantially the same configuration as that of the objective lens210 of the third example, but a second surface (an optical disc sidesurface) thereof is configured to be a single continuous surface. Thatis, the second surface of the objective lens according to the thirdcomparative example is not divided into an inner region and an outerregion.

FIGS. 25A and 25B are graphs illustrating wavefront aberrations causedin an optical system having the objective lens according to the thirdcomparative example when the optical disc 20A is used. FIGS. 26A and 26Bare graphs illustrating wavefront aberrations caused in the opticalsystem according to the third comparative example when the optical disc20B is used. Each of FIGS. 25A and 26A shows the wavefront aberrationregarding an on-axis ray, and each of FIGS. 25B and 26B shows thewavefront aberration regarding an off-axis ray (at an image height of0.06 mm).

FIGS. 27 and 28 respectively show the wavefront aberrations caused inthe third example when the optical discs 20A and 20B are used. Morespecifically, FIG. 27 is a graph illustrating a relationship between thewavefront aberration rms[λ] and the image height [mm] when the opticaldisc 20A is used. FIG. 28 is a graph illustrating a relationship betweenthe wavefront aberration rms[λ] and the image height [mm] when theoptical disc 20B is used.

FIGS. 29 and 30 respectively show the wavefront aberrations caused, inthe optical system including the objective lens according to the thirdcomparative example, when the optical discs 20A and 20B are used. FIG.29 is a graph illustrating a relationship between the wavefrontaberration rms[λ] and the image height [mm] when the optical disc 20A isused in the optical system including the objective lens according to thethird comparative example. FIG. 30 is a graph illustrating arelationship between the wavefront aberration rms[λ] and the imageheight [mm] when the optical disc 20B is used in the optical systemincluding the objective lens according to the third comparative example.

The amount of the wavefront aberration caused when the optical disc 20Ais used is analyzed as follows by making a comparison between FIG. 27and FIG. 29. The coma of the third order caused in the case of the thirdexample is reduced to a level substantially equal to the coma of thethird order caused in the case of the third comparative example.

The amount of the wavefront aberration caused when the optical disc 20Bis used is analyzed as follows by making a comparison between FIG. 28and FIG. 30. The coma of the third order caused in the case of the thirdexample is reduced more sufficiently than the coma of the third ordercaused in the case of the third comparative example. That is, the amountof the wavefront aberration caused in the case of the third example isreduced more sufficiently than the wavefront aberration caused in thecase of the third comparative example.

Therefore, according to the third example, the coma of the third orderis sufficiently suppressed for both of the optical discs 20A and 20B.Consequently, beam spots suitable for recording data to and/orreproducing data from the optical discs 20A and 20B can be formed on thedata recording layers of the optical discs 20A and 20 b, respectively.

An actual shape of the second surface 210 b at the boundary position Pbis smoothed based on the numerical data shown in Tables 11, 12 and 13.Since the objective lens 210 according to the third example satisfiesthe conditions (3) and (4), the smoothed portion is very narrow.Therefore, the smoothed portion does not affect the formation of thesuitable beam spots on the data recording layers of the optical discs20A and 20B. That is, the suitable characteristics shown in FIGS. 23,24, 27 and 28 are attained even if the actual shape of the secondsurface 210 b at the boundary position Pb is smoothed.

Fourth Example

FIG. 31A shows a configuration of an optical system 300 according to afourth example when the optical disc 20A is used. FIG. 31B shows aconfiguration of the optical system 300 according to the fourth examplewhen the optical disc 20B is used. As shown in FIGS. 31A and 31B, theoptical system 300 is configured not to use the collimator lens, so thatcost reduction can be attained. In FIGS. 31A and 31B, to elements whichare the same as those shown in FIGS. 21A and 21B, same reference numbersare assigned.

In contrast to the third example, the first laser beam emitted by thelight source 30 is incident on the objective lens 210 as a divergingbeam, and the second laser beam emitted by the light source 30 isincident on the objective lens 210 as a diverging beam.

Performance specifications of the objective lens 210 according to thefourth example are shown in Table 16.

TABLE 16 First laser Second laser beam beam Design 660 nm 790 nmwavelength Focal length f 3.050 3.072 NA 0.600 0.470 magnification−0.125 −0.124

Table 17 shows a numerical configuration of the optical system 300according to the fourth example when the optical disc 20A is used. Table18 shows a numerical configuration of the optical system 300 accordingto the fourth example when the optical disc 20B is used.

TABLE 17 Surface Number r d n (660 nm) n (790 nm) #0 2.61 #1 ∞ 1.501.51374 1.51052 #2 ∞ 23.00 #3 (h ≦ 1.61) 2.091 3.05 1.54044 1.53653 #3(1.61 ≦ h) 2.131 #4 (h ≦ 1.05) −3.731 1.50 #4 (1.05 ≦ h) −3.768 #5 ∞0.60 1.57961 1.57307 #6 ∞ —

TABLE 18 Surface Number r d n (657 nm) n (790 nm) #0 2.61 #1 ∞ 1.501.51374 1.51052 #2 ∞ 23.36 #3 (h ≦ 1.61) 2.091 3.05 1.54044 1.53653 #3(1.61 ≦ h) 2.131 #4 (h ≦ 1.05) −3.731 1.14 #4 (1.05 ≦ h) −3.768 #5 ∞1.20 1.57961 1.57307 #6 ∞ —

In Tables 17 and 18, “surface number” represents a surface number ofeach surface of optical components in the optical system. In Tables 17and 18, a surface #0 represents the light source 30, surfaces #1 and #2represent a light source side surface and an optical disc side surfaceof the branching element 40, respectively, and surfaces #3 and #4represent the first surface 210 a and the second surface 210 b of theobjective lens 210, respectively. In Table 17, surfaces #5 and #6represent the cover layer and the data recording layer of the opticaldisc 20A, respectively. In Table 18, surfaces #5 and #6 represent thecover layer and the data recording layer of the optical disc 20B,respectively.

As shown in Tables 17 and 18, the first surface 210 a of the objectivelens 210 is divided into the inner region 211 a and the outer region 212a by the boundary position Pa having the height h of 1.61 mm from theoptical axis AX. That is, the inner region 211 a and the outer region212 a are defined as follows.

inner region 211 a: h≦1.61outer region 212 a: 1.61≦h

Similarly, the second surface 210 b of the objective lens 210 is dividedinto the inner region 211 b and the outer region 212 b by the boundaryposition Pb having the height h of 1.05 mm from the optical axis AX.That is, the inner region 211 b and the outer region 212 b are definedas follows.

inner region 211 b: h≦1.05outer region 212 b: 1.05≦h

The first surface 210 a (#3) and the second surface 210 b (#4) of theobjective lens 210 are aspherical surfaces which are defined by theabove mentioned equation. The inner region 211 a and the outer region212 a of the first surface 210 a are configured to be differentaspherical shapes. Also, the inner region 211 b and the outer region 212b of the second surface 210 b are configured to be different asphericalshapes.

Table 19 shows the conical coefficients and aspherical coefficients ofthe first surface 210 a (#3) and the second surface 210 b (#4) of theobjective lens 210 according to the fourth example.

TABLE 19 Surface K A4 A6 #3 (h ≦ 1.61) −0.5000 −4.9090E−03 −3.1240E−04#3 (1.61 ≦ h) −0.5000 −1.5060E−03 −6.4440E−04 #4 (h ≦ 1.05) 0.00003.7715E−02 −1.1230E−02 #4 (1.05 ≦ h) 0.0000 3.6040E−02 −9.7060E−03 A8A10 A12 #3 (h ≦ 1.61) −4.7570E−05 5.2730E−06 −1.6280E−06 #3 (1.61 ≦ h)−3.9280E−05 4.9180E−07 −1.9093E−06 #4 (h ≦ 1.05) 3.5410E−03 −8.7030E−041.0400E−04 #4 (1.05 ≦ h) 2.0640E−03 −3.3203E−04 2.7906E−05

The first surface 210 a of the objective lens 210 has a diffractingstructure expressed by the above mentioned optical path differencefunction Φ(h). In this example, the diffraction order m used for therecording and/or reproducing operation is 1.

Table 20 shows values of the coefficients of the optical path differencefunction Φ(h) applied to the diffracting structure formed within theinner region 211 a and the outer region 212 a of the first surface 210a.

TABLE 20 P2 P4 P6 #3 (h ≦ 1.61) 1.4000E+00 −1.8919E+00 −5.8250E−02 #3(1.61 ≦ h) −2.6204E+00 7.3080E−01 −3.9820E−01 P8 P10 P12 #3 (h ≦ 1.61)0.0000E+00 0.0000E+00 0.0000E+00 #3 (1.61(h) 0.0000E+00 0.0000E+000.0000E+00

With regard to the conditions (3) and (4), the objective lens 210according to the fourth example take values of the distances XA and XB,and the inclinations θ_(2A) and θ_(2B) indicated in Table 21. As shownin Table 21, the objective lens 210 according to the fourth examplesatisfies the conditions (3) and (4).

TABLE 21 Condition (3) Condition (4) X_(A) −0.1162 θ_(2A) −9.99 X_(B)−0.1161 θ_(2B) −10.15 X_(B) − X_(A) 0.0001 θ_(2B) − θ_(2A) −0.16

FIGS. 32A and 32B are graphs illustrating wavefront aberrations causedin the optical system 300 according to the fourth example when theoptical disc 20A is used. FIGS. 33A and 33B are graphs illustratingwavefront aberrations caused in the optical system 300 according to thefourth example when the optical disc 20B is used. Each of FIGS. 32A and33A shows the wavefront aberration regarding an on-axis ray, and each ofFIGS. 32B and 33B shows the wavefront aberration regarding an off-axisray (at an image height of 0.06 mm).

An objective lens according to a fourth comparative example isconsidered as follows for comparing with the objective lens 210according the fourth example. The objective lens according to the fourthcomparative example has substantially the same configuration as that ofthe objective lens 210 of the fourth example, but a second surface (anoptical disc side surface) thereof is configured to be a singlecontinuous surface. That is, the second surface of the objective lensaccording to the fourth comparative example is not divided into an innerregion and an outer region.

FIGS. 34A and 34B are graphs illustrating wavefront aberrations causedin an optical system having the objective lens according to the fourthcomparative example when the optical disc 20A is used. FIGS. 35A and 35Bare graphs illustrating wavefront aberrations caused in the opticalsystem according to the fourth comparative example when the optical disc20B is used. Each of FIGS. 34A and 35A shows the wavefront aberrationregarding an on-axis ray, and each of FIGS. 34B and 35B shows thewavefront aberration regarding an off-axis ray (at an image height of0.06 mm).

FIGS. 36 and 37 respectively show the wavefront aberrations caused inthe fourth example when the optical discs 20A and 20B are used. Morespecifically, FIG. 36 is a graph illustrating a relationship between thewavefront aberration rms[λ] and the image height [mm] when the opticaldisc 20A is used. FIG. 37 is a graph illustrating a relationship betweenthe wavefront aberration rms[λ] and the image height [mm] when theoptical disc 20B is used.

FIGS. 38 and 39 respectively show the wavefront aberrations caused, inthe optical system including the objective lens according to the fourthcomparative example, when the optical discs 20A and 20B are used. FIG.38 is a graph illustrating a relationship between the wavefrontaberration rms[λ] and the image height [mm] when the optical disc 20A isused in the optical system including the objective lens according to thefourth comparative example. FIG. 39 is a graph illustrating arelationship between the wavefront aberration rms[λ] and the imageheight [mm] when the optical disc 20B is used in the optical systemincluding the objective lens according to the fourth comparativeexample.

The amount of the wavefront aberration caused when the optical disc 20Ais used is analyzed as follows by making a comparison between FIG. 36and FIG. 38. Although the coma of the third order caused in the case ofthe fourth example is slightly larger than the coma of the third ordercaused in the case of the fourth comparative example, the amount of thecoma of the third order caused in the fourth example is sufficientlylow. Therefore, the formation of the beam spot suitable for the opticaldisc 20A is not affected.

The amount of the wavefront aberration caused when the optical disc 20Bis used is analyzed as follows by making a comparison between FIG. 37and FIG. 39. The coma of the third order caused in the case of thefourth example is reduced more sufficiently than the coma of the thirdorder caused in the case of the fourth comparative example.

Therefore, according to the fourth example, the coma of the third orderis sufficiently suppressed for both of the optical discs 20A and disc20B. Consequently, beam spots suitable for recording data to and/orreproducing data from the optical discs 20A and 20B can be formed on thedata recording layers of the optical discs 20A and 20 b, respectively.

An actual shape of the second surface 210 b at the boundary position Pbis smoothed based on the numerical data shown in Tables 17, 18 and 19.Since the objective lens 210 according to the fourth example satisfiesthe conditions (3) and (4), the smoothed portion is very narrow.Therefore, the smoothed portion does not affect the formation of thesuitable beam spots on the data recording layers of the optical discs20A and 20B. That is, the suitable characteristics shown in FIGS. 32,33, 36 and 37 are attained even if the actual shape of the secondsurface 210 b at the boundary position Pb is smoothed.

Fifth Example

The configuration of the optical system 200 according to a fifth examplewhen the optical disc 20A is used is shown in FIG. 21A. Theconfiguration of the optical system 200 according to the fifth examplewhen the optical disc 20B is used is shown in FIG. 21B. Performancespecifications of the objective lens 210 according to the fifth exampleare the same as those shown in Table 10.

Table 22 shows a numerical configuration of the optical system 200according to the fifth example when the optical disc 20A is used. Table23 shows a numerical configuration of the optical system 200 accordingto the fifth example when the optical disc 20B is used. The meanings ofthe surface numbers in Tables 22 and 23 are the same as those of thethird example.

TABLE 22 Surface Number r d n (657 nm) n (790 nm) #0 2.36 #1 ∞ 1.501.51383 1.51052 #2 ∞ 17.00 #3 126.470 1.77 1.54056 1.53653 #4 −12.67010.00 #5 (h ≦ 1.57) 2.078 2.21 1.54056 1.53653 #5 (1.57 ≦ h) 2.071 #6(h(1.12) −9.039 1.73 #6 (1.12(h) −9.400 #7 ∞ 0.60 1.57982 1.57307 #8 ∞ —

TABLE 23 Surface Number r d n (657 nm) n (790 nm) #0 2.36 #1 ∞ 1.501.51383 1.51052 #2 ∞ 17.00 #3 126.470 1.77 1.54056 1.53653 #4 −12.67010.36 #5 (h ≦ 1.57) 2.078 2.21 1.54056 1.53653 #5 (1.57 ≦ h) 2.071 #6 (h≦ 1.12) −9.039 1.37 #6 (1.12 ≦ h) −9.400 #7 ∞ 1.20 1.57982 1.57307 #8 ∞—

As shown in Tables 22 and 23, the first surface 210 a of the objectivelens 210 is divided into the inner region 211 a and the outer region 212a by the boundary position Pa having the height h of 1.57 mm from theoptical axis AX. That is, the inner region 211 a and the outer region212 a are defined as follows.

inner region 211 a: h≦1.57outer region 212 a: 1.57≦h

Similarly, the second surface 210 b of the objective lens 210 is dividedinto the inner region 211 b and the outer region 212 b by the boundaryposition Pb having the height h of 1.12 mm from the optical axis AX.That is, the inner region 211 b and the outer region 212 b are definedas follows.

inner region 11 b: h≦1.12outer region 12 b: 1.12≦h

The optical disc side surface (#4) of the coupling lens 50, the firstsurface 210 a (#5) and the second surface 210 b (#6) of the objectivelens 210 are aspherical surfaces which are defined by the abovementioned expression. The inner region 211 a and the outer region 212 aof the first surface 210 a are configured to be different asphericalshapes. Also, the inner region 211 b and the outer region 212 b of thesecond surface 210 b are configured to be different aspherical shapes.

Table 24 shows the conical coefficients and aspherical coefficients ofthe optical disc side surface (#4) of the coupling lens 50, and thefirst surface 210 a (#5) and the second surface 210 b (#6) of theobjective lens 210 according to the fifth example.

TABLE 24 Surface K A4 A6 #4 0.0000 4.3890E−05 2.0480E−07 #5 (h ≦ 1.57)−0.5000 −1.7960E−03 −1.6270E−04 #5 (1.57 ≦ h) −0.5000 −1.2630E−03−2.4790E−04 #6 (h ≦ 1.12) 0.0000 1.5440E−02 −2.2930E−03 #6 (1.12 ≦ h)0.0000 1.4660E−02 −2.1860E−03 A8 A10 A12 #4 8.2010E−10 0.0000E+000.0000E+00 #5 (h ≦ 1.57) 3.7600E−05 −6.8980E−06 7.0714E−06 #5 (1.57 ≦ h)7.0160E−05 −2.4510E−05 3.9525E−06 #6 (h ≦ 1.12) 8.7220E−04 1.4000E−04−5.8565E−05 #6 (1.12 ≦ h) 2.6980E−04 4.1340E−05 −1.0813E−05

The first surface 210 a of the objective lens 210 has a diffractingstructure expressed by the above mentioned optical path differencefunction Φ(h). Table 25 shows values of the coefficients of the opticalpath difference function Φ (h) applied to the diffracting structureformed within the inner region 211 a and the outer region 212 a of thefirst surface 210 a.

TABLE 25 P2 P4 P6 #5 (h ≦ 1.57) 0.0000E+00 −1.6640E+00 −1.3170E−01 #5(1.57 ≦ h) −1.6200E−02 −1.2310E+00 −1.4260E−01 P8 P10 P12 #5 (h ≦ 1.57)0.0000E+00 0.0000E+00 0.0000E+00 #5 (1.57 ≦ h) 0.0000E+00 0.0000E+000.0000E+00

With regard to the conditions (3), (4) and (5), the objective lens 210according to the fifth example take values of the distances X_(A) andX_(B), the inclinations θ_(2A) and θ_(2B) and the inclinations θ_(1A)and θ_(1B) indicated in Table 26. As shown in Table 26, the objectivelens 210 according to the fifth example satisfies the conditions (3),(4) and (5).

TABLE 26 Condition (3) Condition (4) Condition (5) X_(A) −0.0473 θ_(2A)−2.63 θ_(1A) 41.08 X_(B) −0.0472 θ_(2B) −3.17 θ_(1B) 41.02 X_(B) − X_(A)0.0001 θ_(2B) − θ_(2A) −0.54 θ_(1B) − θ_(1A) −0.06

FIGS. 40A and 40B are graphs illustrating wavefront aberrations causedin the optical system 210 according to the fifth example when theoptical disc 20A is used. FIGS. 41A and 41B are graphs illustratingwavefront aberrations caused in the optical system 210 according to thefifth example when the optical disc 20B is used. Each of FIGS. 40A and41A shows the wavefront aberration regarding an on-axis ray, and each ofFIGS. 40B and 41B shows the wavefront aberration regarding an off-axisray (at an image height of 0.06 mm).

FIGS. 42 and 43 respectively show the wavefront aberrations caused inthe fifth example when the optical discs 20A and 20B are used. Morespecifically, FIG. 42 is a graph illustrating a relationship between thewavefront aberration rms[λ] and the image height [mm] when the opticaldisc 20A is used. FIG. 43 is a graph illustrating a relationship betweenthe wavefront aberration rms[ ] and the image height [mm] when theoptical disc 20B is used.

To analyze the amount the wavefront aberration caused in the fifthexample, the third comparative example is used here again. The amount ofthe wavefront aberration caused when the optical disc 20A is used isanalyzed as follows by making a comparison between FIG. 42 and FIG. 29.The coma of the third order caused in the case of the fifth example isreduced to a level substantially equal to the coma of the third ordercaused in the case of the third comparative example.

The amount of the wavefront aberration caused when the optical disc 20Bis used is analyzed as follows by making a comparison between FIG. 43and FIG. 30. The coma of the third order caused in the case of the fifthexample is reduced more sufficiently than the coma of the third ordercaused in the case of the third comparative example.

Therefore, according to the fifth example, the coma of the third orderis sufficiently suppressed for both of the optical discs 20A and 20B.Consequently, beam spots suitable for recording data to and/orreproducing data from the optical discs 20A and 20B can be formed on thedata recording layers of the optical discs 20A and 20 b, respectively.

An actual shape of the second surface 210 b at the boundary position Pbis smoothed based on the numerical data shown in Tables 22, 23 and 24.Since the objective lens 210 according to the fifth example satisfiesthe conditions (3) and (4), the smoothed portion is very narrow.Therefore, the smoothed portion does not affect the formation of thesuitable beam spots on the data recording layers of the optical discs20A and 20B. That is, the suitable characteristics shown in FIGS. 40,41, 42 and 43 are attained even if the actual shape of the secondsurface 210 b at the boundary position Pb is smoothed.

For example, a size of an area (i.e., the smoothed portion LC₂ in FIG.20) for continuously connecting the inner region to the outer region maybe less than or equal to 2% of a sum of areas of the inner and outerregions of the second surface 210 b of the objective lens 210. When thesmoothed portion has such a size, the reduction of the amount of lightby the objective lens can be prevented.

Although the present invention has been described in considerable detailwith reference to certain preferred embodiments thereof, otherembodiments are possible.

In the above mentioned first embodiment, the objective lens isconfigured such that the inner regions correct the coma, caused when theoptical disc 20B is used, more sufficiently than the coma caused whenthe optical disc 20A is used. However, the inner regions of theobjective lens may be configured such that at least the coma, causedwhen the optical disc 20B is used, is reduced. For example, the innerregions of the objective lens may be configured such that the coma issufficiently suppressed for an optical disc whose cover layer has anintermediate thickness between the optical disc 20A and the optical disc20B. Such an objective lens can also attain substantially the sameadvantage as that of the above mentioned embodiment.

In FIG. 20, a size of the smoothed position LC₂ increases as adifference between the distance X_(B) and the distance X_(A) increases.For this reason, it is preferable that the condition (3) is satisfiedwhen the boundary position LC₂ is smoothed.

The objective lens may be formed by injection molding using a moldconfigured such that a portion corresponding to the boundary position ofat least one of the first and second surfaces of the objective lens isprocessed to be a continuous surface by using an R-bite if design shapesof the inner region and the outer region of at least one of the firstand second surfaces are discontinuously connected to each other at theboundary position or are not completely continuously connected to eachother at the boundary position. The R-bite is one of plurality of typesof cutting tools, and has a rounded tip portion.

The optical disc drive employing the objective lens according to theembodiment may be one of a device specialized for recording data tooptical discs, a device specialized for reproducing data from opticaldiscs and a device capable of performing recording operation andreproducing operation.

The present disclosure relates to the subject matters contained inJapanese Patent Applications No. P2003-281814, filed on Jul. 29, 2003,and No. P2003-340755, filed on Sep. 30, 2003, which are expresslyincorporated herein by reference in their entireties.

1. An objective lens comprising a front surface and a rear surface, atleast one surface of the front and rear surfaces including a pluralityof regions having different shapes, the at least one surface isconfigured such that, at each of boundary positions between adjacentones of the plurality of regions, the adjacent ones of the plurality ofregions are continuously connected to each other.
 2. The objective lensaccording to claim 1, wherein the plurality of regions include an innerregion and an outer region outside the inner region, and wherein a sizeof an area for continuously connecting the inner region to the outerregion is less than or equal to 2% of a sum of areas of the inner andouter regions of the rear surface.
 3. An optical system comprising anobjective lens having a front surface and a rear surface, at least onesurface of the front and rear surfaces including a plurality of regionshaving different shapes, the at least one surface is configured suchthat, at each of boundary positions between adjacent ones of theplurality of regions, the adjacent ones of the plurality of regions arecontinuously connected to each other.
 4. The optical system according toclaim 3, wherein the plurality of regions of the objective lens includean inner region and an outer region outside the inner region, andwherein a size of an area for continuously connecting the inner regionto the outer region is less than or equal to 2% of a sum of areas of theinner and outer regions of the rear surface.
 5. An objective lenscomprising a front surface and a rear surface, at least one surface ofthe front and rear surfaces including a plurality of regions havingdifferent shapes, wherein the objective lens is formed by injectionmolding using a mold configured such that portions corresponding toboundary positions of adjacent ones of the plurality of regions areprocessed to be continuous surfaces by using an R-bite.
 6. An opticalsystem comprising an objective lens having a front surface and a rearsurface, at least one surface of the front and rear surfaces including aplurality of regions having different shapes, wherein the objective lensis formed by injection molding using a mold configured such thatportions corresponding to boundary positions of adjacent ones of theplurality of regions are processed to be continuous surfaces,respectively, by using an R-bite.